The vast majority of liquid crystal displays (LCD) require uniform liquid crystal (LC) molecular orientation, usually with a small angle between the LC director n and substrate; this angle is called the “pretilt” angle. A number of methods have been used to achieve tilted alignment of LCs. These are described in detail in Fundamentals and Applications of Liquid Crystals published by Industrial Survey Association (1991). The most common technique to achieve oblique alignment involves deposition of a thin polymer layer on the substrate, which is subsequently rubbed. Rubbing of the polymer determines the azimuthal orientation of the LC molecular alignment, and induces a non-zero pretilt angle. Polyimide (PI) films are commonly used for rubbing alignment of LC's because of their outstanding thermal stability, low dielectric constant, excellent chemical resistance and high productivity. Furthermore, LC alignment on rubbed PI film generally provides a stable pretilt angle preventing reverse tilt disclination of LC molecules with applied voltage. However, the pretilt angle depends on the properties of the orientation film itself. Thus to satisfy specific pretilt angle requirements for various LCD modes, specific polyimides have been made for controlling the pretilt angle. For example, polyimides with long alkyl and fluorinated alkyl side groups have been used to generate high LC pretilt angles. It has been suggested that steric interaction between LC molecules and branched long alkyl side chains is a possible cause for high pretilt angles.
The rubbing method suffers from several drawbacks, however, especially accumulation of static charges at the thin film transistor sites and generation of dust particles. Recently, new non-rubbing alignment techniques, based on photo-induced anisotropy of the polymerizable orienting layers, have been introduced. Typically the photosensitive polymer films are illuminated by polarized ultraviolet light, and the azimuthal orientation of the resulting planar alignment depends on the specifics of the photo-induced reaction. In contrast to the rubbing technique, neither excess charge nor dust is created on the substrates, yet control is maintained over both the tilt angle and the anchoring strength. The traditional rubbing technique establishes a unique direction of the tilted easy axis; this direction is determined by the direction of rubbing. On the other hand, for photoalignment there is a twofold degeneracy of the light-induced easy axis. This twofold degeneracy causes poor reproducibility of the pretilt angle and, more importantly, the appearance of defects at the resulting boundaries between orientation domains. This degeneracy may be partially removed during the filling of the LC cell because of the effect of flow alignment, but the resulting alignment is not temporally stable. To date, the most promising method to break this degeneracy involves oblique irradiation of the photoalignment layer. Oblique polarized irradiation makes an angle with the surface and the photoreaction for on-axis transition moments is much easier than that of off-axis ones. Consequently the tilt degeneracy is broken and the liquid crystals tilt in a preferred direction. Such an irradiation scheme requires specialized equipment and have proven difficult to implement in a large scale process.
Other non-contact for aligning LC molecules include a stretched polymer, a Langmuir Blodgett film, a grating structure produced by microlithography, oblique angle deposition of silicon oxide, and ion beam irradiation of a polyimide surface as in U.S. Pat. No. 5,770,826. The method places the LC's on a polyimide surface which has been bombarded with low energy (about 100 eV) Ar+ ions. This method has been extended to include diamond-like carbon (DLC), amorphous hydrogenated silicon, SiC, SiO2, glass, Si3N4, Al2O3, CeO2, SnO2, and ZnTiO2 films as described in U.S. Pat. No. 6,020,946.
JP 2002038158 discloses a method for the formation of a liquid crystal layer containing liquid crystal molecules on a substrate and the orientation of the liquid crystal molecules. A pyridinium quaternary salt is added to the liquid crystal layer or a layer adjacent to it, and the inclination angle of the liquid crystal molecules is controlled by the action of the pyridinium quaternary salt. Although this invention provides an advantage in controlling the tilt angle of liquid crystal over other existing methods, it only provides a limited class of molecules that are capable of increasing the tilt; thus, further new materials for inducing LC pretilt are needed.
In all the methods of LC alignment described above, control of LC pretilt angle requires the use of a specific combination of the LC molecules and the alignment polymer or specific materials. Developing and optimizing such combination (of alignment polymers and LC's) is a difficult and time-consuming process. There is a need for alternative ways to control the pretilt angle of liquid crystal to the desired angle and in an easy manner.